Science - USA (2020-07-10)

INSIGHTS | PERSPECTIVES

sciencemag.org SCIENCE

PHOTO: SHERRI AND BROCK FENTON

ECOLOGY

Bats navigate with cognitive maps

Tagging and tracking systems reveal the way-finding strategies of fruit bats

By M. Brock Fenton

A

cognitive map can allow an animal
to navigate from its current posi-
tion to an undetected goal. There is a
long-standing, ongoing debate about
which animals have and use cognitive
maps (1–3). On pages 188 and 194 of
this issue, Toledo et al. ( 4 ) and Harten et
al. ( 5 ), respectively, show that Egyptian
fruit bats (see the figure) use cognitive
maps, as evidenced by taking previously
unused shortcuts. These are movements
between two known sites that are beyond
detectable range of one another. Shortcuts
are strong evidence of cognitive maps.
Toledo et al. used ATLAS, a new
high-throughput tracking system, to
document the travels and home ranges
of 172 tagged Egyptian fruit bats over
3449 bat-nights across 4 years. Each
tag weighed 4 g (<4% of the animal’s
body mass) and provided over 18 mil-
lion localizations. The ATLAS system
simultaneously tracked dozens of
animals with high resolution and ac-
curacy. Using direct observations and
translocation experiments, Toledo et
al. showed that to get to fruit trees,
tagged bats repeatedly used goal-
directed, long, straight flights rather
than random searches. Tagged bats
also commonly took shortcuts to go
directly to fruit trees. The researchers
used trajectory analyses and time-lag
embedding to rule out nonmap strat-
egies. Their analyses revealed that the
tagged bats did not systematically follow
known routes, nor did they directly sense
cues such as landmarks or beacons. These
animals relied on a cognitive map frame of
reference for their current positions in rela-
tion to a goal that they had not yet detected.
Some Egyptian fruit bats have home ranges
of over 100 km^2 , and earlier research revealed
that tagged animals typically flew directly (in
a straight line) to specific fruit trees within
their home ranges ( 6 ). But, as Toledo et al.
cautioned, they lack data about the early ex-
perience of the animals that they tracked.
Harten et al. addressed this shortcom-
ing by establishing an in-house colony of
Egyptian fruit bats on campus. By day, bats

in the colony roosted on site and, at night,

freely foraged in the surrounding area. The

researchers collected data using continuous

Global Positioning System tags (0.99 g, or

0.42 g) affixed to the bats. Tag readers in the

roost and at nearby sites downloaded data

about the bats’ comings and goings and their

specific routes. Harten et al. focused on data

from the flights of 22 Egyptian fruit bat pups

from their initial flights outdoors through the

first months of their lives. These bats gradu-

ally increased the sizes of their home ranges,

after 70 days reaching typical adult areas of

60 km^2. Over time, after feeding at local trees

within their home ranges, young animals

sometimes made exploratory flights that in-

volved going well beyond their normal home

ranges before returning to the roost.

Young bats also took shortcuts, identi-

fied by conservative criteria. To be consid-

ered a shortcut, at least 50% of the trajec-

tory of a movement had to be original, and

the destination at least 100 m from any

other site the bat had previously visited.

These stringent criteria reduced the num-

bers of recorded shortcuts.

Like other bats, Egyptian fruit bats are

long-lived (often over 20 years in the wild),

have low reproductive output (usually one

young per litter), large neonate size (25%

of mother’s mass), and often roost in social

groups ( 7 , 8 ). Adult masses range from 80

to 170 g, making them well suited for work

involving relatively large tags. Advances

in the capacity of tags, such as those used

in either study, have greatly increased our

knowledge of bats. Further embellishments

involve tags that record video and audio

from free-flying bats ( 9 ). In addition, prox-

imity tags ( 10 ) provide new insight into so-

cial interactions of bats. Researchers have

used active tags on bats for over 50 years,

typically keeping tag size to <5% of bat body

mass ( 11 ). Yet, most of the ~1400 species of

bats weigh <50 g as adults, putting many, if

not most, beyond the range of current tag

technology ( 12 ). Documenting the home

ranges of bats will prove central to their

conservation, which might mean setting

aside and protecting larger areas.

The hallmarks of these two groups of re-

searchers include innovative use of

tags, thorough and consistent analy-

ses, and demanding criteria, as well as

exhaustive long-term field work and

strong collaborations. Their results

convincingly show that Egyptian fruit

bats navigate using cognitive maps.

Together, both studies ( 4 , 5 ) advance

our knowledge and understanding of

cognitive maps and open the door to

learning how widespread this cogni-

tive behavior may be among animals.

Further advances in tag technology

will expand our knowledge of how bats

learn their home ranges, from nursery

colonies to hibernation sites. Such de-

velopments will put other findings in

broader perspective such as bats’ use

of food resources with predictable dis-

tribution in space and time. Details

about social organization and interac-

tions also can further elucidate bats’ roles as

disease vectors ( 13 , 14 ). j

REFERENCES AND NOTES

1. E. C. Tolman, Psych. Rev. 55 , 189 (1948).


2. D. R. Griffin, Animal Minds (Univ. of Chicago Press, 1992).


3. M. Geva-Sagiv, et al., Nat. Rev. Neurosci. 16 , 94 (2015).


4. S. Toledo et al., Science 369 , 188 (2020).


5. L. Harten et al., Science 369 , 194 (2020).


6. A. Tsoar et al., Proc. Natl. Acad. Sci. U.S.A. 108 , E718 (2011).


7. G. Kwiecinski, T. Griffiths, Mamm. Species 611 , 1 (1999).


8. R. Cohen, Rousettus aegyptiacus , Animal Diversity Web.
   accessed 2 June 2020; https://animaldiversity.org/
   accounts/Rousettus_aegyptiacus/ (2011).
   9 L. Stidsholt et al., Methods Ecol. Evol. 10 , 48 (2019).


9. S. P. Ripperger et al., Curr. Biol. 29 , 1 (2019).


10. H. D. J. N. Aldridge, R.M. Brigham. J. Mammal. 69 , 379
    (1988).


11. M. B. Fenton, N. B. Simmons, Bats: A World of Science and
    Mystery (Univ. of Chicago Press, 2015).


12. K. M. Bakker et al. Nat. Ecol. Evol. 3 , 1697 (2020).


13. M. B. Fenton et al., Nat. Ecol. Evol. 4 , 517. (2020).

10.1126/science.abd1213

Department of Biology, The University of Western Ontario,

London, Ontario, Canada. Email: [email protected]

Egyptian fruit bats

(Rousettus aegyptiacus)

range widely from South

Africa north to Turkey,

developing shortcuts to

fruit trees where they feed.

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